This invention relates to power transistors with semiconductor bodies.
When shutting off a power transistor, local fusing of the semiconductor body and resulting destruction of the transistor can occur, if the permissible operating range of the transistor is exceeded. This permissible operating range is characterized by a maximally permissible characteristic switching power loss which must not be exceeded in the semiconductor body for a certain period of time. The maximally permissible power becomes lower, the longer it is converted to heat in the semiconductor body. The reason for the local fusing when the permissible operating range is exceeded presumably is that, in shutting off the transistor, the current setting in and flowing laterally under the emitter biases the blocked emitter-base-pn-junction in the forward direction so that a high current can flow at simultaneously rising base-collector voltage. These difficulties have been overcome in the past by designing power transistors with fingershaped emitter zones in which the lateral resistance below the emitter zone is reduced in accordance with the narrower width of the fingers. Accordingly, the permissible operating range of such a transistor is greater when compared to a transistor of the same, but contiguous area.
Another known solution to overcome these problems consists of dividing a power transistor into a multiplicity of partial transistors integrated on a single semiconductor body and connected to a common emitter electrode via a ballast resistor. Such a solution is described, for instance, in the journal "Electronic Engineering", July 1983, pages 78-79, FIG. 15. The emitter zones of each partial transistor are embedded in a base zone common to all partial transistors. The ballast resistor of each partial transistor is formed by a zone of opposite conduction type embedded in the emitter zone. While this avoids direct contact between ballast resistor and the base zone, the lateral dimensions of the emitter zone cannot fall below a certain size, however. Therefore, a strong current flowing laterally under the emitter zone can bias the base-emitter-pn-junction positively, and the pn-junction starts emitting.
Accordingly, the invention relates to a power transistor with a semiconductor body containing at least one base zone, at least one collector zone and a multiplicity of emitter zones connected to an emitter electrode via a resistor each.